It is known that fire tube condensing boilers have become widespread; they have a structure which comprises an outer enclosure, which is adapted to limit a portion of space designed to contain the water to be heated, which is fed cold by means of an intake coupling located at the base and flows out hot through a discharge coupling located proximate to the top of such enclosure.
The enclosure contains, in a raised position, the furnace where the combustion takes place, and tubes branch out from such furnace and are designed to convey the flue gases which transfer heat to the water to be heated before they are sent to the stack; such tubes can equally be oriented vertically or subvertically, i.e., with a certain inclination.
All the described components are advantageously made of carbon steel, which differently from what occurs for stainless steel ensures an easy procedure for joining the various components by welding; but in this way, such components are subjected to the danger of corrosion by the condensation which, since this is a condensing boiler, is generated in the flue gases as a consequence of the condensation of the water vapor contained in the flue gases.
It is further necessary to consider the presence of other heat exchange devices, such as economizers, which comprise vertical or subvertical flue gas conveyance tubes which are connected in an upward region to a tube plate made of carbon steel.
A co-pending patent application by the same Applicant is related to protection against the danger of corrosion of the flue gas conveyance tubes.